I. Field of the Invention
The present invention relates to a tilt sensor for detecting a tilt of an object when the object is tilted.
II. Description of the Prior Art
The present inventors have proposed a tilt sensor in which a permanent magnet having a surface coated with a magnetic fluid is housed in a hollow nonmagnetic case, and a magnetic sensing element responsive to magnetism of the permanent magnet is disposed outside a bottom portion of the case (U.S. patent application No. 112,626, and E.P.C. Application No. 87115618.8).
The above prior art will be described first with reference to FIG. 6.
Referring to FIG. 6, reference numeral 1 denotes a nonmagnetic case made of, e.g., aluminum. As shown in FIGS. 7A and 7B, the hollow portion of the case 1 comprises a bottom portion 2 having a circular flat surface 2a constituting its central portion as a first bottom surface, and an annular inverted frustum surface 2b constituting its peripheral portion as a second bottom surface defining a predetermined angle .theta. with the flat surface 2a. FIG. 7A is a sectional view taken along the line A--A in FIG. 7B. Reference numberal 3 denotes a substantially disk-like permanent magnet having its surface coated with a magnetic fluid 4. The magnetic fluid 4 is prepared by dispersing magnetic metal powder (e.g., cobalt, iron, or nickel powder) particles having a diameter of 50 .ANG. to 150 .ANG. in a liquid (e.g., kerosene or water) having a relatively low viscosity. Generally, a surface-active agent is added to the magnetic fluid 4. Reference numeral 5 denotes a cover disposed at a predetermined height from the bottom portion 2 so as to prevent the permanent magnet 3 from being turned upside down; 6, a mounting base; and 7, a reed switch serving as a nonlinear magnetic sensing element disposed outside the bottom portion 2 of the case 1. One end portion of one lead 7a is positioned near the central portion of the bottom portion 2. One end portion of the other lead 7b is positioned away from the bottom portion 2.
An operation of this prior art will be described below. FIG. 7B shows a state wherein the case 1 is not tilted. The flat surface 2a of the bottom central portion is horizontal. In this case, since the permanent magnet 3 is located on the flat surface 2a, one end portion of one lead 7a of the reed switch 7 is located near the permanent magnet 3. As a result, one end portion of the lead 7a is influenced by the magnetism of the permanent magnet 3, and hence is polarized to the N pole, whereas the other end thereof (an intermediate portion of the reed switch) is polarized to the S pole. Furthermore, since one end portion of the other lead 7b located near the S pole is polarized to the N pole, both the reeds attract each other and are connected. That is, if a circuit is connected to the leads 7a and 7b, this circuit is closed.
Assume that the case 1 is tilted in a given direction, as shown in FIG. 8. When a tilt angle of the case 1 is small, since the frustum surface 2b is still ascending at a certain angle with respect to a horizontal plane X, the permanent magnet 3 is located near the central flat portion. Therefore, the lead 7a of the reed switch 7 is influenced by the magnetism of the permanent magnet 3, and hence both the reeds are held in a connected state.
As shown in FIGS. 9A and 9B, when the case 1 is further tilted at an angle .theta. defined by the flat surface 2a of the substantially central portion and the frustum (in practice, at an angle slightly larger than .theta., as shown in FIG. 9C, because of the viscosity of the magnetic fluid), and the frustum surface 2b becomes a descending surface, the permanent magnet 3 is quickly and completely moved from the central flat surface 2a to the frustum surface 2b by its weight. That is, the position of the permanent magnet 3 is nonlinearly changed with respect to the central flat surface 2a. When the permanent magnet is moved, since the magnetic fluid 4 serves as a lubricating oil, the resistance to the movement is very small so that the permanent magnet 3 can be easily moved to the lowest position of the bottom portion. In this case, one end of one lead 7a of the reed switch 7 is quickly moved out of the substantially influential range of the magnetism of the permanent magnet 3, i.e., nonlinearly moved out of the range. As a result, the magnetization is substantially lost, and the magnetic attraction between the leads 7a and 7b is eliminated, thereby opening the switch.
As is apparent from the above description, in the prior art, when the case is tilted at an angle larger than a predetermined angle, the reed switch is quickly, i.e., nonlinearly opened. With this operation, it can be clearly detected that the case is tilted at an angle larger than the predetermined angle. In addition, when the case is returned to its original position, the permanent magnet is also returned to the central flat surface portion, so that the reed switch is quickly, i.e., nonlinearly closed.
However, since the magnetic flux of the permanent magnet 3 constitutes a magnetic path running through the reed switch 7 and returning to the permanent magnet 3, the viscosity of the magnetic fluid 4 is reduced in the above-described prior art. In addition, a force for attracting the permanent magnetic 3 to the reed switch 7 is generated, and hence the permanent magnet 3 tends to be urged against the bottom portion 2 of the case 1. For this reason, even if the tilt of the tilt sensor is changed, the permanent magnet 3 may not be smoothly moved on the bottom portion 2. Another drawback is that the movement of the permanent magnet 3 tends to be delayed with respect to tilting of the tilt sensor.